Generally, an optical card has very high information recording density, as compared with other recording cards such as a magnetic card, an IC card and so on. The optical card has an optical data recording section on one side of a card substrate. Data is recorded on the optical data recording section by optical energy, chiefly, a laser beam. The recorded information is optically reproduced due to a difference in light reflectivity between the recorded portion and the non-recorded portion.
The general background of such optical card is disclosed in, for example, U.S. Pat. Nos. 4,680,460 and 4,673,626. In the conventional optical card, the optical data recording section is formed by one of various materials such as a low melting-point metal such as tellurium, bismuth or the like, silver chloride photographic material, a metal having high reflectivity such as aluminum, and so on.
By the way, the optical cards are divided broadly into two categories. One of them is a read-only optical card, that is, a ROM (Read Only Memory) optical card in which a user performs only reading. The other is an additional recording optical card, that is, a so-called WO (Write Once) optical card in which the user performs also writing. With respect to the reading, it is preferable that the optical data recording section in the optical card has high light reflectivity. Accordingly, in the ROM optical card, metals of high reflectivity such as aluminum and so on are used, to form a pattern forming material of on the optical data recording section. In this connection, in such ROM optical card, the pattern forming material generally presents a silver color and, accordingly, the design and appearance of the card tends to be monotone. For this purpose, Japanese Patent Provisional Publication Number SHO 62-298036 has proposed a technique in which a light colored layer is provided at a location adjacent the optical data recording section to enhance the design.
On the other hand, in the WO optical card, while performing reading, also requires consideration of the writing characteristic. For this reason, in the WO card, material issued which has light reflectivity of 40%, 50% or the like, normally, 35%.about.55%. In this connection, U.S. Pat. No. 4,230,939 discloses a technique in which, when the optical data recording section is made of material having relatively high reflectivity such as bismuth, aluminum or the like, an absorption layer containing dyes is provided on the optical data recording section in order to reduce writing energy due to the laser beam.
As described above, generally, since different pattern forming materials are used in the ROM optical card and the WO optical card there is considerable difference in light reflectivity between the pattern forming materials. An optical card such as an ROM card which uses a metal having high reflectivity such as aluminum, its alloy or the like as the forming material of the pattern, requires an expensive special purpose machine for reading the card. The ROM optical card can be produced in large quantity by an etching technique or the like which employs a photo mask, so that an attempt can be made to reduce the cost. In contradistinction thereto, it has been considered that a problem of the read-only special purpose machine forms a serious obstacle in the spread and acceptance of the card. In this connection, in case of both the WO optical cards, or in case of both the ROM cards, it is difficult to use a common reader, if the two cards are considerably different in light reflectivity, for example, if the light reflectivity exceeds 10%.
It is not known in the art to have optical cards each having a respective light reflectivity caused to coincide per se with each other. In any of the aforementioned Japanese Patent Provisional Publication Number SHO 62-298036 and U.S. Pat. No. 4,230,939, the technique is disclosed in which the colored layer or the absorbing layer is provided in the front side of the light reflecting surface, but no way of thinking is disclosed in which the light reflectivities, especially, the light reflectivities with respect to the reading light are made substantially equal to each other.